The invention relates to a method of making iron in a melter-gasifier-reduction furnace system wherein the reduction gas and the stripped top gas produced in the system are recycled. Specifically the invention relates to an improvement to an iron making process using a reduction furnace and a melter gasifier for making hot metal, in which the top gas produced in the reduction furnace during the reduction of iron ore is stripped of CO.sub.2 and is preheated and combined with reduction gas from the melter gasifier and the combined gas is injected into the reduction furnace for reduction of iron ore.
In recent years methods utilizing a melter gasifier have been developed to produce molten iron or steel preproducts and reduction gas. Most of these processes utilize a coal fluidized-bed. A high temperature is produced in the melter gasifier utilizing coal and blown in oxygen to produce a fluidized bed and iron sponge particles are added from above to react in the bed to produce the molten iron.
A melter gasifier is an advantageous method for producing molten iron or steel preproducts and reduction gas as described in U.S. Pat. No. 4,588,437. Thus there is disclosed a method and a melter gasifier for producing molten iron or steel preproducts and reduction gas. A first fluidized-bed zone is formed by coke particles, with a heavy motion of the particles, above a first blow-in plane by the addition of coal and by blowing in oxygen-containing gas. Iron sponge particles and/or pre-reduced iron ore particles with a substantial portion of particle sizes of more than 3 mm are added to the first fluidized-bed zone from above. A melter gasifier for carrying out the method is formed by a refractorily lined vessel having openings for the addition of coal and ferrous material, openings for the emergence of the reduction gases produced, and openings for tapping the metal melt and the slag. Pipes or nozzles for injection of gases including oxygen enter into the melter gasifier above the slag level at least two different heights.
Another process utilizing a melter gasifier is described in U.S. Pat. No. 4,725,308. Thus there is disclosed a process for the production of molten iron or of steel preproducts from particulate ferrous material as well as for the production of reduction gas in the melter gasifier. A fluidized-bed zone is formed by coke particles upon the addition of coal and by blowing in oxygen-containing gas by nozzle pipes penetrating the wall of the melter gasifier. The ferrous material to be reduced is introduced into the fluidized bed. In order to be able to produce molten iron and liquid steel preproducts in a direct reduction process with a lower sulfur content of the coal used, the ferrous material to be reduced is supplied closely above the blow-in gas nozzle plane producing the fluidized bed. An arrangement for carrying out the process includes a melter gasifier in which charging pipes penetrating its wall are provided in the region of the fluidized-bed zone closely above the plane formed by the nozzle pipes. The ferrous material to be melted as well as the dusts separated from the reduction gas and, if desired, fluxes containing calcium oxide, magnesium oxide, calcium carbonate and/or magnesium carbonate are introduced therethrough.
U.S. Pat. No. 4,739,855 to Rolf Hauk discloses a process for the gasification of sewage sludge or other carbon-containing waste materials in a gasifier. A solid fuel and oxygen-containing gas are also fed into the gasifier. The solid fuel may be coal or petroleum coke. The residues formed during gasification collect at the bottom of the gasifier in the form of molten slag. Gasification takes place in a fluidized bed formed above the slag bath and constituted by the dried sewage sludge or waste materials, the solid fuel, the oxygen-containing gas and the gasification gas. The gas produced in the gasifier can be used for power generation or as a reducing gas for iron ore. Sponge iron can simultaneously be melted in the gasifier and reduced to pig iron.
There is also a process that utilizes a combined reduction furnace and melter gasifier known as the COREX.RTM. process (COREX.RTM. is a trademark of Deutsche Voest-Alpine Industrieanlagenbau GMBH and Voest-Alpine Industrieanlagenbau). This process is described in Skillings'Mining Review, Jan. 14, 1989 on pages 20-27 and in detail in the Prior Art section of this specification. Broadly in the COREX.RTM. process the metallurgical work is carried out in a reduction furnace and a melter gasifier. Using non-coking coals and iron bearing materials such as lump ore, pellets or sinter, hot metal is produced with blast furnace quality. Passing through a pressure lock system, coal enters the dome of the melter gasifier where destructive distillation of the coal takes place at temperatures in the range of 1,100.degree.-1,150.degree. C. Oxygen blown into the melter gasifier produces a coke bed from the introduced coal and results in a reduction gas consisting of 95% CO +H.sub.2 and approximately 2% CO.sub.2. This gas exits the melter gasifier and is dedusted and cooled from about 1100.degree. C. or so to the desired reduction temperature between 800.degree. and 850.degree. C. The gas is then used to reduce lump ores, pellets or sinter in the reduction furnace to sponge iron having an average degree of metalization above 90%. The sponge iron is extracted from the reduction furnace using a specially designed screw conveyor and drops into the melter gasifier where it melts to the hot metal. As in the blast furnace, limestone adjusts the basicity of the slag to ensure sulfur removal from the hot metal. Depending on the iron ores used, SiO.sub.2 may also be charged into the system to adjust the chemical composition and viscosity of the slag. Tapping procedure and temperature as well as the hot metal composition are otherwise exactly the same as in a blast furnace. The top gas of the reduction furnace has a net calorific value of about 7,000-8,000 KJ/Nm.sup.3 and can be used for a wide variety of purposes.
The fuels used in these processes are typically described as a wide variety of coals and are not limited to a small range of coking coal. The above-noted article from Skillings'Mining Review notes that petroleum coke suits the requirements of the COREX.RTM. process. Brown coal and steam coal which are relatively poor quality coal having a relatively high ash content i.e. plus 15%, have been identified as suitable for use in these processes. Coke made from coal has also been identified as a fuel for many of the processes utilizing melter gasifiers.
The present invention is directed to an improvement in the method of making iron in a melter-gasifier-reduction furnace.